Treatment of water with soluble dehydrated sodium phosphates



Patented Se t. 26, 1944 UNITED STATES PATENT OFFICE TREATMENT OF WATERWITH SOLUBLE DEHYDRATED SODIUM PHOSPHATES' Charles B. Durgin, Robert N.Foster, and Charles F. Booth, Anniston, Ala., assignors to MonsantoChemical Company, :St. Louis, Mo., a corporation of Delaware ApplicationJuly 2, 1934, Serial No. 733,392

Claims.

. the provision of a water-treating compound which shall have pronouncedproperties for the retention in-solution of the common alkalineearthmetals and compounds thereof.

It has long been known that mixtures of the various ortho-sodiumphosphates could be used to establish a definite hydrogen ionconcentration .in aqueous solutions. This has been made use of in thetreatment'of boiler waters and of detergent solutions where thephosphate radicle served as a precipitating agent to precipitate thepositive ion of the various undesirable salts of the alkaline earthgroup'metals from solution, this effect taking place in a solution whichwas maintained at a hydrogen ion concentration which was not injuriousto the boiler steel, or

which was suitable for the various detergent 30 uses in which such saltswere employed. In some cases it is desirable to employ as a watertreating agent pyro and meta phosphates which will give the desiredalkalinity to boiler water and at the same time exhibit a delayedprecipitating action towards the alkaline earth metal salts present innatural waters. The pyro and meta phosphates are especially well suitedfor this purpose. since they do not immediately hydrate to theorthophosphates and hence do not immediately precipitate insolubleortho-phosphates. For some purposes a greater delay in the rapidity ofthe reversion of such meta or pyro phosphates to the ortho-phosphates isdesirable and it is a further additional object of the present inventionto provide a dehydrated phosphate product, which exhibits an increasedresistance to reversion.

We have found that mixtures of tetrasodium pyro-phosphate and sodiummeta phosphate fused together and chilled to form an amorphousglass-like soluble, dehydrated phosphate product are particularlysuitable for the present purpose. By varying the constituentsof theproduct any desired hydrogen ion concentration between the limits ofpH=6.4 to pH=10.2 may be obtained.

We have furthermore found that readily soluble mixtures of molecularlydehydrated sodium phosphates of a predetermined hydrogen ionconcentration may be obtained by heating to the fusion pointappropriately proportioned mixtures of sodium ortho-phosphates, ormixtures of orthophosphoric acid and the sodium salt of a replaceableacid and then rapidly cooling. Suitable sodium salts for this purposeare the chloride and carbonate of sodium, although others may be used.The latter compound is preferable since it is difiicult, when producingmixtures having a high alkali content to expel all of the chlorine fromthe melt.

In the accompanying drawings, Figure 1 shows certain solubilityrelationships of the sodium and potassium series of dehydratedphosphates. Figure 2 shows the hydrogen ion concentration of the sodiumphosphates prepared by our. process. Figure 3 shows the hydrogen ionconcentration of the dehydrated potassium phosphates. Figure 4illustrates the solubility of the sodium and potassium series ofphosphates in concentrated solutions. In Figure 5 is given the meltingpoints of the sodium phosphates.

Our invention will be illustrated by the follow ingexamples:

Example 1 Mix together 76.5 lbs. of mono-sodium orthophosphate(NaH2PO4.2H2O) and 134.7 lbs. of di-' cool-quickly by pouring the moltenmass on a cold metal surface in thin layers. The layers of molten saltshould not be over inch thick and preferably less than /2 inch thick.The metal surface on which the molten salt is poured, if insubstantially continuous use as in the case of continuously operatedchill rolls, may be water cooled in order that accumulated heat may becarried away rapidly. We have found that water at ordinary temperaturesis preferable. However, it is possible to operate satisfactorily whencooling with water at or near its boiling point.

We have found that it is particularly desirable, in order to develop theamorphous condition, to cool rapidly during the first portion of thecooling period. vFor example, the maximum benefit from quick cooling isobtained by cooling from the melting point to say in the neighborhood ofto 300 C. If this is done rapidly, then it is not necessary that theremainder of the cooling be at so rapid a rate. For the purpose of thisspecification, we shall designate as quickly cooling," both the processof cooling at a rapid unbroken rate from the original to the finaltemperature, and the process of cooling at a rapid unbroken rate fromthe original to an intermediate temperature, followed by cooling at adifferent rate (either faster or slower) than the original rate.

The product from the melt as above designated, Example 1 isapproximately 100 lbs. of a solid consisting theoretically of 50%tetrasodium pyro-phosphate and 50% sodium meta phosphate. A sample ofthis material as produced in commercial operation and dissolved in waterto make a 1% solution will show a'pH of approximately 8.6. See curve Cof Figure The reactions for stoichiometrically determining thetheoretical proportions in this example are: q

Example 2 Mix together 74.14 lbs. of sodium carbonate (soda ash) and78.26 pounds of phosphoric acid, H3PO4 (or 104.2 lbs. 'of 75% H3PO4).Heat slowly ina gas fired reverberatory furnace as long as carbondioxide and water vapor are evolved, then raise the temperaturegradually to the melting point, which is approximately at a temperatureof 940 C. Heating may be carried out in a carbon crucible or on afirebrick hearth heated by radiation. When a uniform melt has beenobtained, cool the melt quickly by pouring it on a cooled metal surfaceas described in the previous example. Approximately 100 lbs. of amixture having a theoretical composition consisting of 80% tetra sodiumpyro-phosphate and 20% of sodium meta phosphate will be obtained. Asample of this material as produced in commercial operation anddissolved in water so as to make a 1% solution will show a pH in theneighborhood of 9.5. See curve C of Figure 2.

The reactions for stoichiometrically determining the theoreticalproportions in this example are:

The examples above illustrate two ways by which two of our improveddehydrated phosphates may be made. Obviously any one method might beused for both mixtures, as well as other mixtures not specificallymentioned. In general,

it may be said that the results of both methodsapplied to any onemixture yield a very similar product, hence it becomes an economicquestion of choice of raw materials as to which method is employed.

The proportioning of the raw materials is done on a simplestoichiometric basis as indicated which is familiar to persons skilledin the art.

ing point, being presumablysuper-cooled liq'ulds.

Mixtures containing stoichiometrically from 50% to 100% of tetra-sodiumpyro-phosphate and 50% to sodium meta phosphate, exhibit a graduallyincreasing crystalline fracture and a sharper freezlngpoint, the mixtureat the same time becoming less and less transparent as the theoreticaltetra-sodium pyro-pho'sphate content approaches When carrying out ourprocess on an industrial scale, heating of the phosphate salts ormixtures of suitable salts and phosphoric acid, is preferably carriedout in a reverberatory furnace with gas heating. In such cases we havefound that the product formed by our process contains a relatively smallproportion i. e. less than 5% ortho and acid pyro-phosphate. .Thepresence of this impurity in our fused mixtures is probably due to thepresence of water vapor in the combustion gases and also, in part, dueto the rapidity of the heating and melting operation as carried out onsuch a scale. The presence of this small amount of impurity isimmaterial in the various uses in which the product is employed, itsefiect being merely to change somewhat the hydrogen ion concentration ofthe solutions which are prepared therefrom.

For this reason we have shown on Figure 2 of the accompanying drawingstwo curves labelled C and C. Curve C has been prepared from the sodiumphosphates by first carefully dehydrating the material in an electricfurnace at temperatures at which the meta phosphate is known to form andthen raising the temperature to the fusion point of the mixture. Curve Cis representative of material which has been industrially prepared byfusion in the presence of .water vapor in an industrial furnace. Byvarying the conditions of dehydration of the ortho-phosphates somevariation can be made in the acidity relations of the products formedtherefrom. It may be said that in general the range of hydrogen ionconcentrations obtained by products produced according to our processwill lie between the values given by curves C and C on Figure 2.

In the above description we have referred to the composition ascontaining theoretically or stoichiometrically a certain amount ofsodium meta phosphate and tetra-sodium pyro-phosphate. We have used thisterminology because although the stoichiometric proportions are basedupon the presence of these two compounds, a chemical examination of ournew product indicates that these compounds have in some cases lost theirchemical identity, while in other cases such chemical identity has beenextensively altered.

This may be illustrated by the following test.

If a solution be prepared by dissolving in water a fused mixturecontaining the 60% sodium meta phosphate and 40% tetra-sodiumpyro-phosphate equivalent and the solution then tested for pyro-.phosphate content according to the method of Britske and Dragunov, J.Chem, Ind. Moscow, 4, 4951 (1927) Chem. Abstr. 22 2900 (1928), asurprising result is obtained.

On the addition of zinc sulphate to the neutralized solution noprecipitate is formed. On the other hand if a mechanical mixture asdistinct from a fused mixture and having the same theoreticalproportions as that given above is similarly tested, a heavy precipitateof zinc pyrophosphate is obtained. This test shows-that zincpyro-phosphate can be precipitated from a solution of the mechanicalmixture while no such Y assaoes 3 I precipitate is formed in a solutionof the tus'ed mixture. This indicates that the pyro-phosphate contenthas lost its chemical identity in the fused mixture, and while suchmixture contains the stoichiometric alkali and acid equivalent of thepyro-phosphate it no longer behaves chemically as if such pyro-phosphatewere present.

In the same manner it can be demonstrated that in certain of thecompositions the meta phosphate has also lost its identity. This may bedeter-.

mined by testing solutions of mechanical mixtures of the sodium metaphosphate and tetrasodium pyro-phosphate in comparison with fusedmixtures of the same equivalent composition.

The solutions of the mechanical mixtures of these salts will givecharacteristic heavy precipitates of meta phosphate with silver nitrateand barium chloride. The solutions of the fused mixtures show either noprecipitate or only a small amount, depending on the composition.

These chemical tests indicate that fused mixtures of the two salts nolonger contain metaphosphate or pyro-phosphate as such and hence differfrom known compositions of the prior art.

An important distinction between our compositions and known compositionsor compositions which mightordinarily be considered as equivalentsthereof resides in the ready solubility of our product as compared withsuch other compositions. Such a property is important since one of theimportant applications of our product has to do with the treatment ofwater for various purposes.

The types of solubility effect will be illustrated, i. e. thepreparation of dilute solutions and the preparation of concentratedsolution.

Dilute solutions Such solutions of our dehydrated and ,fused sodiumphosphates may be prepared by dissolving a proportion of the phosphatedirectly in water. For example a 1% solution is prepared by adding 1% ofthe solid dehydrated sodium phosphate to 99% of water. Any and all ofthe phosphates prepared according to our process and ranging inequivalent composition between say 99% sodiummetaphosphate and 1%tetra-sodium pyro-phosphate to 1% sodium meta phosphate and 99%tetrasodium pyrophosphate can be utilized in this manner to form a 1%solution, as shown in lineAof Figure 1.

As contrasted with this property of the sodium salts, we have found thatthe dehydrated and.

fused potassium salts do not form 1% solutions in all proportionsthroughout'the series. If 1% of the corresponding solid dehydratedpotassium salts are placed in contact, with 99% of water, the solidphase will not dissolve completely. The extent of solubility of thedehydrated potassium phosphate series is shown as line B in Figure 1.The hydrogen ion concentration of the 1% solution of the sodium salts isshown in Figure 2 while the same data are given in Figure 3 forsuchsolutions of the potassium salts as were formed under similarconditions. It will be understood, .however, that the latter data arenot given for 1% solutions because of the impossibility of forming suchsolutions.

Concentrated solutions Saturated solutions of these dehydrated sodiumphosphates were prepared by mixing together water andiany of the sodiumphosphate compositions herein described in .such proportions thatequilibrium has been attained. We have found that the compositi in ofthe solution differed from some solid phase is present in the solutionafter 76 that of the solid phase in equilibrium with said solution. Thisindicates a differential or selective solubility effect in which one ormore constituents of the solid tend to dissolve while others tend toremain undissolved.

The nature of this solubility effect is shown on Figure 4 by curve E.From these data it will be seen that high solubility i. e., over 50%total solids is obtained in the dehydrated sodium phosphate series inthe range between about 99 and slightly less than 50% equivalent sodiummeta phosphate content, while if less than 50% sodium meta phosphate ispresent the solubility of the salt rapidly declines,'reaching a minimumof approximately 6% for the compound Na4P2O1.

In the case of the P tassium series the solubility effect as measured bythe total solid dissolved in concentrated solutions is exactl reversed.In this casesalt mixtures having a high content of potassium metaphosphate are relatively insoluble, the solubility of the seriesincreases as the content of tetra-potassium pyrophosphate increases andreaches the highest value of the series for the pure compound, K4P2O7.The

solubility of the series of dehydrated potassium phosphates is given oncurve F of Figure 4.

In connection with our experiments on the differential or selectivesolubility of the dehydrated sodium and potassium phosphates we havemade the following additional observations:

Selective solubility in the dehydrated phosphate series sodiumpyro-phosphate equivalent is in no case completely disolved.

In the potassium series the conditions are exactlyv reversed. In thisseries the pyrophosphate constituent of the fused mixtures is completelydissolved throughout most of the range of compositions. The potassiummeta phosphate constituent is in no case completely dissolved. .Suchproperties are of importance. as indicating a definite absence ofequivalency in the sodium and potassium series of fused dehydratedphosphates.

It has previously been proposed to utilize mixtures of thepyro-phosphate and meta phosphate for the treatment of water for boileruse. Such mixtures, however, present the following objections.

The mechanical or loose mixtures are difiicult to maintain in uniformcondition throughout the mass of the mixture. meta phosphate isobtained, as fairly large pieces resembling broken glass while thepyro-phosphate is either in crystalline form or in a finely groundcondition. It is undesirable to grind the materials because the formeris hygroscopic and readily becomes sticky and gummy if exposed to air ofordinary humidity. Even though a uniform powder could be prepared at thefactory, shipment and the attendant handling of the mixture before Thisis so because theuse would tend to segregate one material from the massand make the preparation of a. definite solution of the material adiflicult one.

We have furthermore determined that our fused homogeneous mixtures havea lower pH than mechanical or loose mixtures having the same originalstoichiometric composition. For example a fused mixture containing theequivalent of 80% meta and tetra pyro-phosphate in 1% solution has a pHof 7.5, a 60% meta tetra-pyrophosphate has a DH of 8.0 while a.40% meta60% tetrapyrophosphate has a pH of 8.7. The pH of correspondingmechanical mixtures have values of. 8.6; 9.1 and 9.4 respectively. Fromthe above values we see that the fused mixtures have a greater aciditythan the mechanical mixtures of from 0.7 to 1.1 pH units, all of whichpoints'to a greater alkali neutralizing capacity inherent in ourimproved product.

Further study of our improved product shows that we obtain a greaterbuffering action, in other v words a greater reserve acidity than isobtained by simple mixtures of the meta phosphate andtetra-pyro-phosphate. This is confirmatory of the findings mentionedabove which indicate a greater alkali neutralizing capacity. Thebuffer-. ing property is of value in other ways than-those mentioned andthis property will be found of value by those skilled in the art.

A further valuable property possessed by our .fused dehydrated sodiumphosphates is the enhanced water softening properties. Since it hasalready been proposed to soften water utilizing the meta phosphate, theproperties of such phosphate in preventing the precipitation of thealkaline earth metal compounds are more or less well known. We have nowfound that solutions formed from fused mixtures, the equivalent of metaand pyro-phosphate in stoichiometric pro portions, have the property ofproducing zero hardness using a considerably lesser proportion axialangle.

"axial and positive and have a small The indices are:

Alpha= 1.473 plus or minus 0.002 Gamma=1.494 plus or minus 0.002

. The crystals are orthorhombic, platy in shape,

and have a negative elongation.

Composition stoichiometrically equivalent to a content of approximatelypercent of sodium meta phosphate and 50 percent of tetra-sodiumpyro-phosphate are mostly isotropic amorphous solids having an index ofrefraction of 1.480 plus or minus 0.002. There are a few anisotropic,bi-

axial crystals visible in the isotropic amorphous solld." Compositionsranging in content between the 50-50 composition mentioned above and acomposition containing the stoichiometric equivalent of 90 percentsodium meta phosphate and 10 percent of tetra-sodium pyro-phosphate,have the same refractive index as that mentioned for the 50-50composition and show fewer crystalline inclusions.

Compositions of our products ranging between the stoichiometricequivalent of v50 percent sodium meta phosphate and 50 percent oftetra-sodium pyro-phosphate to the 90 percent tetra.- sodiumpyro-phosphate and 10 percentsodium meta phosphate compositions show agradually increasing anisotropic biaxial crystalline content in the'amorphous solid. The crystalline phase appears as plate-like or featherycrystals with the amorphous isotropic material between the crystals. Theisotropic material has an index of 1.480. Thecrystalline phase has theoptical properties of the tetra-sodium pyro-phosphate given abovethat isthe crystals are anisotropic and biaxial.

of our phosphate product than of an equivalent mechanical mixture.

Since it has been proposed to prepare solutions of mixtures of the twoconstituents namely the meta and tetra-pyro-phosphate, the relativerates of solution of correspondingmixtures of our fused product are ofimportance. In comparative tests with our fused homogeneous product andmechanical mixtures of equivalent stoichiometric composition we havefound that our improved phosphates dissolve completely and in arelatively short time while the mechanical mixtures of equivalentcomposition require an inordinately long time for complete solution.

We have also noted, in this connection, that our product causes a lesserdegree of corrosion on the dissolving equipment. This is believed to bedue to the fact that theconstituents of our product dissolve completelyat a uniform rate provided suificient water is present to form acomplete solution, we have also noted that the acidity of the solutionis at no time excessively high due to the acid constituent dissolving ata faster rate.

This is a factor of considerable importance because of the universal useof iron dissolving equipment in water treating operations.

Optical properties of the dehydrated sodium phosphates The opticalexamination of our improved product indicates therefore that thosecompositions having a major proportion ofequivalent sodium metaphosphate consist of a homogeneous amorphous isotropic solid having arefractive index between the limits of 1.478 and 1.482 in whichisotropic solid is suspended a relatively small proportion ofanisotropic crystalline material.

The proportion of crystalline material in our product is present insmaller proportion than that indicated by the stoichiometric compositionof the mass. This clearly indicates that a chemical combination of theconstituents present has occurred which combination results in theformation of a new compound characterized by the properties given above.Such a conclusion agrees with the chemical properties which have alreadybeen mentioned.

Because of'the complexity of the dehydrated sodium phosphates obtainedby our process we are unable to more accurately define them. Since theyare definitely reproducible by following the methods outlined in ourdisclosure above they are properly defined and'characterized by themethods stated.

;The products herein described embody the socalled polyphosphates asdescribed in Gmelins Handbuch 8th ed. volume Natrium" pages 924- 925.For exampleusing proportionsof sodium salts such that one obtains thestoichiometric equivalent, in the fused product of 1 mol of Na4P2O1 and1 mol of NaPO4, the cooled fused product will contain the sodium salt oftriphosphoric acid, namely thecompound NasPaOm. In similar manner onemay obtain the compound sodium tetraphosphate, NaeP4O 1a, theproportions being such that the fused product contains the equivalent of1 mol of Na4PzO-1 and 2 mols of NaPOa. This compound is a salt oftetraphosphoric acid,

H6P401s. The compound sodium penta phosphate Nal2Pl0O13 may likewise bemade by combining by fusion 1 mol of Na4P2Ov and 8 mols of NaPOz, thiscompound being the salt of penta phosphoric acid, H12P10013.

Intermediate products between the compounds specifically enumerated maybe made by suitably choosing the pro-portions of the starting materials.For example instead of producing the pure compound, N36P40l3'0l16 may byincreasing the proportions of meta phosphate equivalent employed obtaina product containing the compound NasP4O13 plus some meta phosphate. Thesame applies to products containing the constituent sodiumtetrapyrophosphate and sodium penta phosphate.

These compounds may be used for water softening and as washing aids oringredients because they tend to sequester the alkaline earth ion, as isdescribed by'Hall in U. S. Patent 1,956,515 in the case of sodium hexameta phosphate alone. The result of such sequestration or combination isto eliminate or reduce the relatively large concentration of freecalcium or other alkaline earth ions present in the hard water..

In view of the fact that water may be effectively softened by the use ofalkali metal salts of the polyphosphoric acids, the compounds hereindescribed may be used in a variety of industries. The compounds may forexample be employed as water softeners in boiler plants, in delimingleather, in dyeing textiles, scouring wool, kier boiling, silkdegumming, general washing purposes with or without soap especiallywhere hard water ordinarily precipitates an in-' soluble compound. Theymay also be incorporated into soap to produce a soap composition havingwater softening properties.

Having now particularly described our invention and the manner in whichit may be worked, it will be apparent that our invention is susceptibleof various changes and modifications without departing from the spiritthereof, and we desire therefore that our invention be not limitedexcept as indicated by the prior art or as particularly pointed out inthe claims.

What we claim is: 1. A process which comprises adding to hard water, soas to combine with an alkaline earth characterized in that its power toprevent deposition of said alkaline earth metal compound is greater thanthat of an unreacted mixture of sodium meta and tetrasodiumpyrophosphate.

'2TA process which comprises adding to hard water, so as to combine withan alkaline earth metal compound therein, to prevent deposition of saidcompound, the fused reaction product of monosodium and disodiumorthophosphate, to produce when dissolved in water an aqueous solutionhaving a, pH between 8 and 10, the weight proportions of NaPOs, derivedfrom said monosodium orthophosphate, and Na4PzOv, derived from disodiumorthophosphate in said reaction product, falling within the range of50:50 and 20:80 respectively, said reaction product being furthercharacterized in that its power to prevent deposition of said alkalineearth metal compound is greater than that of an unreacted mixture ofsodium metaphosphate and tetrasodium pyrophosphate.

3. A process which comprises adding to hard water, so as to combine withan alkaline earth metal compound therein to prevent deposition of saidcompound, the fused reaction product of monosodium and disodiumorthophosphate, to produce when dissolved in water, an aqueous solutionhaving a pH between 9 and 10, the molecular proportions of NaPO: derivedfrom monosodium orthophosphate, and Na4PzOw derived from disodiumorthophosphate in said heat reaction product being substantially 1 to 1,said V heat reaction product being further characterized in that itspower to prevent deposition of said alkaline earth metal compound isgreater than that of an unreacted mixture of sodium metaphosphate andtetrasodium pyrophosphate.

'4. A process which comprises adding to hard water, so as to combinewith an alkaline earth metal compound therein to prevent deposition ofsaid compounds, the water soluble fused reaction product of soda ash andorthophosphoric acid, to produce when dissolved in water an aqueoussolution thereof having a pH between 8 and 10; the theoretical weightproportion of sodium metaphosphate and tetrasodium pyrophosphate in saidreaction product falling within the range of :50 and 20:80 respectively,said reaction product being further characterized in that its power toprevent deposition of said alkaline earth metal compound is greater thanthat of an unreacted mixture-of sodium metaphosphate and tetrasodiumpyrophosphate.

5. A process which comprises adding to hard water, so as to combine withan alkaline earth metal compound therein to prevent deposition of saidcompound, the water soluble fused reaction product of soda ash andorthophosphoric acid to produce when dissolved in water an aqueoussolution thereof having a pH between '7 and 10, the theoretical weightproportion of sodium metaphosphate and tetrasodium pyrophosphate in saidreaction product falling'within the range 20:80 and :20 respectively.said heat reaction product containingv a proportion of sodium acidpyrophosphate,- and being characterized further in that its power toprevent deposition of said alkaline earth metal compounds is greaterthan that of an unreacted mixture of sodium metaphosphate andtetrasodium pyrophosphate.

CHARLES B. DURGIN. ROBERT N. FOSTER. CHARLES F. BOOTH.

